Luminary talks backwards (4)
I believe the answer is:
star
'luminary' is the definition.
(I've seen this before)
'talks backwards' is the wordplay.
'talks' becomes 'rats' (I've seen this in another clue).
'backwards' shows that the letters should be reversed in order.
'rats' written backwards gives 'STAR'.
(Other definitions for star that I've seen before include "It twinkles" , "E.g.Sirius" , "eg The sun" , "Distant sun" , "Eg, Antares" .)